Single mothers may face poorer health in midlife than married mothers, according to a recent national study published in the American Sociological Review. The finding is particularly notable, the authors said, considering that about 40% of all births in the U.S. now occur to unmarried women, up from 10% in 1960.

The study found that women who were not married when they had their first child reported worse health by age 40, compared with first-time mothers who were married. “We are soon going to have a large population of single mothers who are entering midlife, when many health problems just begin to emerge,” Kristi Williams, an associate professor of sociology at Ohio State University and a lead author of the study, said in a statement. “This is a looming public health crisis that has been pretty much ignored by the public and by policy makers.”

Why single motherhood predicted poor health wasn’t entirely clear, but the researchers think it has to do with the stress of raising a child alone, as well as economic challenges faced by many single moms. The findings suggest these stressors may have long-term effects on physical health. “Research has clearly shown the toll that long-term stress takes on health, and we know that single mothers have a great deal of stress in their lives,” Williams said. “Their economic problems only add to the problem.”

Williams and her team used data from the National Longitudinal Survey of Youth, involving more than 4,500 women who were interviewed annually from 1979 (when they were aged 14-22) until 1994, and every two years since then. By 2008, the researchers had data on marriages and other relationships for a 29-year period.

Although single mothers overall tended to report worse health by middle age, the researchers found some differences along ethnic and racial lines: Hispanic women who had a first child out of wedlock appeared to be protected from the same adverse health effects that affected whites and blacks, for instance. That may be due to the fact that unwed Hispanic mothers tended to have children with committed long-term partners, researchers said; Latino families are also traditionally large and tight-knit and include adult children — all factors that provide a crucial support network for single mothers. In turn, that may reduce women’s stress and help preserve their good health.

One factor that did little to reverse the negative health effects of single motherhood was marriage. For most women, getting married or entering a long-term relationship after becoming a single mom didn’t improve health outcomes. “It is a tall order to expect that marriage can counteract the cumulative strains of unwed motherhood and their eventual negative impact on health,” Williams said.

However, in some cases when women married their children’s biological father and stayed married, their health at 40 benefited. Researchers said marriage in these cases offered women social and financial support. But while these rewards were seen by whites and Hispanics, they were not experienced by black women, who tended to have more out-of-wedlock births than the other groups.

That later marriage was no antidote to the health effects associated with single motherhood was noteworthy, when considered against the fact that welfare reforms in 1996 and 2006 included authorizations to expand programs to promote marriage among low-income single mothers, based on the assumption that marriage improves health.

The researchers concluded:

More research is needed on the mechanisms through which single motherhood undermines the health of U.S. women and on the way these processes unfold over the life course and, possibly, across generations. Chronic financial strain likely plays an important role.

But the authors note that the cohort in the study was of a generation in which birth out of wedlock was not only less socially acceptable than it is today, but also tended to happen earlier in life — among teens or younger women with few resources. These factors, which can contribute to stress, social isolation and poverty, may help explain the adverse health effects reported. But it’s possible that many women who are having children outside of marriage today may not suffer the same problems.